Clonazepam's primary mechanism of action is via modulating GABA function in the brain, via the benzodiazepine receptor which in turn leads to enhanced GABAergic inhibition of neuronal firing. In addition clonazepam decreases the utilisation of 5-HT (serotonin) by neurons and has been shown to bind tightly to central type benzodiazepine receptors. Because of its strong anxiolytic and anticonvulsant properties, it is said to be among the class of "highly potent" benzodiazepines. The anticonvulsant properties of benzodiazepines are due to inhibition of postsynaptic GABA responses and inhibition of sustained high frequency repetitive firing.
Benzodiazepines, including clonazepam, bind to mouse glial cell membranes with high affinity. Clonazepam decreases release of acetylcholine in cat brain and decreases prolactin release. Benzodiazepines inhibit cold-induced thyroid stimulating hormone (also known as TSH or thyrotropin) release. Benzodiazepines acted via micromolar benzodiazepine binding sites as Ca2+ channel blockers and significantly inhibited depolarization-sensitive calcium uptake in an experiment on rat brain cell components. This has been conjectured as a mechanism for high-dose effects on seizures in the study.
The primary mechanism of action of clonazepam is binding to benzodiazepine receptors, which causes an enhancement of GABA binding. This results in inhibitory effects on the central nervous system. Benzodiazepines, however, do not have any effect on the levels of GABA in the brain. Clonazepam has no effect on GABA levels and has no effect on gamma-aminobutyric acid transaminase. Clonazepam does however affect glutamate decarboxylase activity. It differs insofar from other anticonvulsant drugs it was compared to in a study. Benzodiazepine receptors are found in the central nervous system and have been identified in a wide range of peripheral tissues such as longitudinal smooth muscle-myenteric plexus layer, lung, liver and kidney as well as mast cells, platelets, lymphocytes, heart and numerous neuronal and non-neuronal cell lines.
It has shown itself to be highly effective as a short-term (3 weeks) adjunct to SSRI treatment in obsessive-compulsive disorder and clinical depression in reducing SSRI side effects with the combination being superior to SSRI treatment alone in a study funded by the manufacturers of clonazepam, Hoffman LaRoche Inc. Similar results have been found with some other anxiety disorders, but the role of the serotonergic effects enhancing the action of the SSRI treatment remains unclear in these cases due to clonazepam's primary anxiolytic mechanism of action.
Peak blood concentrations of 6.5–13.5 ng/mL were usually reached within 1–2 hours following a single 2 mg oral dose of micronized clonazepam in healthy adults. In some individuals, however, peak blood concentrations were reached at 4–8 hours.
Clonazepam passes rapidly into the central nervous system with levels in the brain corresponding with levels of unbound clonazepam in the blood serum. Clonazepam plasma levels are very unreliable amongst patients. Plasma levels of clonazepam can vary as much as tenfold between different patients.
Clonazepam is largely bound to plasma proteins. The metabolites of clonazepam include 7-aminoclonazepam, 7-acetaminoclonazepam and 3-hydroxy clonazepam.
In humans tolerance to the anticonvulsant effects of clonazepam occurs frequently. Chronic use of benzodiazepines leads to the development of tolerance with a decrease of benzodiazepine binding sites. Clonazepam has also been shown to produce effects of myosis, or "pinpointing" of the pupil. The degree of tolerance is more pronounced with clonazepam than with chlordiazepoxide. Short term therapy is generally more effective than long term therapy with clonazepam for the treatment of epilepsy. Many studies have found that tolerance develops to the anticonvulsant properties of clonazepam with chronic use, which limits its long term effectiveness as an anticonvulsant.
Discontinuation of or reduction in dosage after regular use may result in the clonazepam withdrawal syndrome. Abrupt or over-rapid withdrawal from clonazepam may result in the development of the benzodiazepine withdrawal syndrome with psychotic attacks characterised by dysphoric manifestations, irritability, aggressiveness, anxiety, and hallucinations. Sudden withdrawal from clonazepam may also result in withdrawal symptoms including anxiety, irritability and potentially the life threatening condition status epilepticus. Antiepileptic drugs, benzodiazepines such as clonazepam in particular, should be reduced slowly and gradually when discontinuing the drug to reduce withdrawal effects. Carbamazepine has been trialed in the treatment of clonazepam withdrawal and has been found to be ineffective in preventing clonazepam withdrawal status epilepticus from occurring.
Clonazepam may be prescribed for:
In the treatment of acute epilepsy via intravenous administration approximately 72.5 per cent of patients show improved EEG patterns, 17.5 per cent show no improvement and for 10 per cent of patients clonazepam has a paradoxical effect and worsens EEG readings.
Clonazepam is sometimes used for refractory epilepsies; however, long-term prophylactic treatment of epilepsy has considerable limitations, most notably the loss of antiepileptic effects due to tolerance, which renders the drug useless with long-term use, and also side effects such as sedation, which is why clonazepam and benzodiazepines as a class should generally only be prescribed for the acute management of epilepsies.
Clonazepam or diazepam has been found to be effective in the acute control of nonconvulsive status epilepticus. However, the benefits tended to be transient in many of the patients and the addition of phenytoin for lasting control was required in these patients.
Clonazepam has been found to generally be ineffective in the control of infantile spasms. Clonazepam is less effective and potent as an anticonvulsant in bringing infantile seizures under control compared with nitrazepam in the treatment of West syndrome, which is an age-dependent epilepsy affecting the very young. However, as with other epilepies treated with benzodiazepines, long-term therapy becomes ineffective with prolonged therapy, and the side effects of hypotonia and drowsiness are troublesome with clonazepam therapy; other antiepileptic agents are therefore recommended for long-term therapy, possibly Corticotropin (ACTH) or vigabatrin. Clonazepam is therefore not recommended for widespread use in the management of seizures related to West syndrome.
Clonazepam has been used in the management of seizure disorders in children and also for infantile spasms. However, usefulness of clonazepam is limited due to its dose limiting side effects, especially its negative effect on cognition.
Clonazepam was approved in the United States as a generic drug in 1997 and is now manufactured and marketed by several companies.
Clonazepam is available in the U.S. as tablets (0.5, 1.0, and 2 mg) and orally disintegrating tablets (wafers) (0.125, 0.25, 0.5, 1.0, and 2 mg). In other countries, clonazepam is usually available as tablets (0.5 and 2 mg), orally disintegrating tablets (0.25, 0.5, 1 and 2 mg) oral solution (drops, 2.5 mg/mL), as well as solution for injection or intravenous infusion, containing 1 mg clonazepam per ampoule (e.g. Rivotril inj.).
Benzodiazepines such as clonazepam can be very effective in controlling status epilepticus but when used for longer periods of time serious side effects may develop such as interference with cognitive functions and behaviour. Many individuals treated on a long-term basis develop a form of dependence known as "low-dose dependence", as was shown in one double-blind, placebo-controlled study of 34 therapeutic low-dose benzodiazepine users—physiological dependence was demonstrated via flumazenil-precipitated withdrawal. Use of alcohol or other CNS depressants while taking clonazepam greatly intensifies the effects (and side effects) of the drug. Side effects of the drug itself are generally benign, but sudden withdrawal after long-term use can cause severe, even fatal, symptoms.
Caution in children. Clonazepam is not recommended for use in those under 18. Use in very young children may be especially hazardous. Of anticonvulsant drugs behavioural disturbances occur most frequently with clonazepam and phenobarbital.
Caution using high dosages of clonazepam. Doses higher than 0.5 - 1 mg per day is associated with significant sedation.
Clonazepam may aggravate hepatic porphyria.
Caution in schizophrenia. Clonazepam has been found to be not effective in the management of schizophrenia and has been found to increase the risk of violent behavior.
Coma can be cyclic with the individual alternating from a comatose state to a hyperalert state of consciousness, as occurred in a 4-year-old boy who suffered an overdose of clonazepam. The combination of clonazepam and certain barbiturates eg amobarbital at prescribed doses has resulted in a synergistic potentiation of the effects of each drug leading to serious respiratory depression.